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Parallel Op-amps

  The arrangement of op-amps shown below may seem a bit odd but the circuit effectively increases the drive capability while reducing noise over a single op-amp with essentially no loss in performance.

Schematic for a parallel op-amp configuration for increased drive current.

    Each op-amp in this circuit is configured as a buffer. In this configuration their outputs will very closely match each other. Any imbalance will, however, cause them to drive each other rather than the load. The resistors shown mitigates this problem. With only minor differences in output from each op-amp (typically in the mV range) a small resistance value can effectively balance the outputs by limiting the amount of current from each op-amp used to overcome the imbalance.

    A good starting point for the value of R is in the 5Ω to 10Ωrange. To see how this works, assume there is an imbalance between two amplifiers of 10mV. The 5Ω resistor means that 1 mA of drive current is required to resolve the imbalance with the remaining output current available to drive the load (E = I * R, 10mV = I * (2 * 5), I = 1 mA).

    The parallel values of the resistors is seen by the load which minimizes their effect on the circuit. For instance, a quad op-amp and 5Ω resistors will appear to have an internal resistance of 1.25Ω from the loads perspective.

    The noise figure in this configuration is also reduced.  While it may not seem obvious, adding random noise sources reduces the average value of the noise by the square root of the number of noise sources added together.  If we assume that the output of four op-amps were tied together, the noise would be reduced by a factor of 2.  Given the low value of the resistors involved, they contribute a negligible amount of noise to the output.

copyright © 2021 John Miskimins